The present invention lies in the field of cellulose acetate polymers, blends, and other compositions. In addition, the invention lies in the field of methods for making such compositions.
Cellulose acetate is a widely-used plastic that can exhibit a variety of chemical and physical properties depending upon its chemical structure. For example, by varying the degree of acetyl substitution on the cellulose backbone, the product can range from a low degree of crystallinity to considerable crystallinity. And the degree of substitution also dictates the product's solubility: a completely substituted product (triacetate) dissolves in dichloromethane but not in acetone, while a somewhat less substituted product dissolves in acetone but not in dichloromethane. Further, by substituting propionate or butyrate for acetyl groups, the flexibility and moisture resistance of the polymer can be increased. The presence of sulfate and certain other inorganic substituents on the cellulose backbone causes the cellulose acetate to become less stable (and more biodegradable).
Consistent with its varied properties, cellulose acetate is used in various commercial products such as textile fibers, photographic films, lacquers, sheeting, and bulk structures. Rayon, the widely-used fiber, is high-purity cellulose acetate that has been spun or extruded from an acetone solution. Non-fibrous, cellulose acetate articles may be prepared by injection molding or extrusion. In some cases, cellulose acetate products contain additives such as plasticizers or other plastics.
The properties of the cellulose acetate are dictated by the reaction conditions under which cellulose is reacted with acid anhydride. In conventional processes, cellulose starting material is esterified with acetic anhydride in the presence of a catalyst such as sulfuric acid and a solvent such as acetic acid. In some cases, other solvents such as chloroform or dichloromethane are added to solubilize the cellulose acetate in the early stages of the reaction. If the reaction goes to completion, i.e. nearly all of the free hydroxy groups on the cellulose starting material are esterified, a triacetate will form. Of course, the degree of acetyl substitution will vary with the reaction time, the reaction temperature, and other parameters. Further, water can be added to the reaction mixture to promote hydrolysis of the ester groups during a "ripening period." This typically converts the triacetate into a less acetylated, acetone-soluble form that can be used to spin rayon fibers.
To make cellulose acetate for rayon, a high-purity source of cellulose is required. If impurities are present at even low concentrations, extrusion may become difficult or impossible. Thus, clean, high-grade cotton linters (produced by milling operations) are typically used in commercial processes. Cotton linters contain small diameter fibers which can be rapidly converted to a reactive acid-soluble form.
Unfortunately, cotton linters are not an ideal starting material for all cellulose acetate products. Sometimes waste seeds, milling oils and other contaminants have to be separated from the useful cotton material. And because cotton is usually grown in warmer climates, transportation (as low-density, loose bales) to factories in colder areas adds to the cost of the final product. Further, an increasing percentage of cotton linters have naturally colored fibers as opposed to white fibers; yet white or clear products are often preferred for plastic products.
The high cost of cotton linters (depending on the quality, between $0.18 and $0.22/pound) makes cellulose acetate uneconomical for many applications. In 1992, bulk quantities of cellulose acetate cost about $1.65/pound (excluding transportation costs), a relatively high price in comparison to petroleum and gas derived polymers such as polyethylene, polystyrene, polypropylene and polyvinyl chloride. As a result, cellulose acetate has been displaced by less expensive polyethylene and similar polymers for many applications. Yet, for some of these applications, a high-purity, high-cost cellulose acetate polymer is not needed. For instance, many molded plastic products can be produced from cellulose acetate containing considerable filler or other impurities.
Given the high cost of most commercial cellulose acetate made from cotton linters, it can be seen that an inexpensive source of cellulose acetate is desirable for various applications.